Titanium-based materials are useful because of their relatively low weight and high strength over a wide range of operating temperatures. Titanium or titanium alloys often are the material of choice for high performance gas turbine engine components, such as the fan rotor, fan blades, compressor disk and compressor blades. A wide range of titanium alloys are available, each of which confer a particular combination of characteristics to the component. For example, some gas turbine compressor blades are made of the commercially available Ti-6A1-4V alloy.
The efficiency of gas turbine engines is dependent, in part, on the ability of the above titanium-based and other engine components to combine air and products of combustion, to intended pathways. Leakage from such design flowpaths can reduce engine performance and efficiency and thus gas turbine engine designers have developed a variety of sealing arrangements, such as abradable seals, to work in conjunction with other components to reduce or control leakage. For example, abradable seals are used on the shrouds of compressors to insure efficient operation of the engine by minimizing gas leakage in the compressor and turbine sections. Although the engine is typically designed and manufactured to precise dimensional tolerances, centrifugal and thermal expansion of the rotating and stationary members makes it difficult to achieve zero clearances. Thus, abradable seals often are employed on surfaces of the stationary member allowing penetration of rotating blades into the seal.
Some modern gas turbine engines employ titanium alloy rotor blades and an AlSi coated seal. During operation, the titanium alloy blades may contact and rub into casings or adjacent hardware coated with AlSi. The condition of the blade after operation can vary depending on the severity of the rub. Severe rubs may result in over heating of the blade tip with a range of conditions varying from an associated color tint on the tip, transformation of the blade tip microstructure to deformation of the blade tip. However, there are no visual indications for blades that experienced mild contact into the AlSi coating resulting in the deposit of aluminum on the blade. Such deposit onto the blades is aerodynamically undesirable and may result in decreased engine performance and efficiency. Thus, if a blade rub is suspected, all of the blades are removed from the engine and replaced.
Accordingly, there exists a need for a nondestructive inspection technique that identifies the presence of aluminum-based material deposited on titanium-based components during engine operation. The present invention satisfies this need.